1. Field of Invention
The invention relates to a method for the continuous production of boards of ligneous material, and an apparatus for the practice of the process.
2. Description of the Related Art
Ligneous materials comprising fibers, chips or strands can be manufactured quite effectively into boards with the use of organic binding agents. Organically bound adhesives (urea, melamine and phenol formaldehyde) and isocyanates are used predominantly, as well as mixtures of these binding agents. Boards which are bonded with urea resin and with modified urea resin are cooled after hot pressing. The cooling has proven necessary, since after curing in the hot press, the urea resin hydrolyzes by the action of temperature and moisture (xe2x80x9chydrolysis effectxe2x80x9d). The hydrolysis of the binding agent leads to weakening of the adhesive bond and thus to an impairment of the practical properties of the ligneous material boards. Thus, the mechanical properties, such as resistance to flexure and transverse strength are impaired and swelling of the board is increased.
Cooling is performed in various ways. As a rule, the boards are transported after hot pressing to a cooling star turner [Kxc3xchlsternwender]. Transport from the end of the press to the cooling star turner takes approximately two minutes. The boards are fanned out in the turner. Air has free access to the boards on all sides. With warming of the air, the boards are slowly cooled. Usually several turners are used to cool the boards, which pass through one after the other. After about 20 minutes, the temperature at the center of the board has dropped to about 70xc2x0 C. The thickness of the board is reduced during cooling by up to 0.2 mm, a large part of the thickness reduction being caused by the drying out of the boards (post-shrinkage). Since transport to the first cooling star turner takes about two minutes and the boards cool slowly, the adhesive bonds formed in the hot press are partially destroyed. This weakening of the adhesive bond happens mainly with adhesives that are sensitive to hydrolysis.
The moisture distribution over the board""s cross section becomes varied during the hot pressing and cooling. During the hot pressing, moisture is vaporized out of the outer layer and carried to the middle of the board where part of the vapor condenses. Some of the vapor travels from the middle of the board to the edges of the board and passes into the atmosphere, so that the average moisture decreases by one to three percent during the hot pressing. Immediately after the hot press, it is mainly the surface layers that dry out, so that the average moisture content of the board is further reduced. After cooling, the moisture in the surface layers is about 2% and in the middle layer about 6-8%. The boards are then stacked in the warehouse. In the curing warehouse, the board cools down further and an equalization of moisture takes place within the board. In addition, the board absorbs moisture from its environment. Often some boards become warped in storage due to uneven climatic conditions in the stack. Considerable capital is tied up in such a warehouse, and this entails expense. Even after three weeks of storage, the moisture is not uniformly distributed throughout the board""s cross section. Further, the average moisture content of the board is below the equalization moisture of 8% in countries of moderate climate and 12% in countries of humid climate.
Various other proposals have been made for cooling the boards. An attempt has been made to cool the boards in cooling channels with forced ventilation. The use of cooling, channels proved to be disadvantageous since the climatic conditions were controllable to only a limited degree. This resulted in irregular temperature distributions being created in the boards. It has also been proposed to cool the boards in conditioning chambers (Kioseff Holzindustrie 1974). In this case, the boards were set up vertically and passed through a chamber in the first part of which the boards are cooled with air with a natural rising air current. In the second part, forced air currents are passed around the boards. Even in this conditioning chamber, the air velocity was not uniform, so that temperature differences occurred in the board upon cooling, resulting in warping in the boards. The cooling time could not be appreciably shortened in comparison to the star turners.
Cyclic presses have also been used for the cooling and for later equalization of boards. Such cyclic presses have been set up at some distance from the hot press in plants with single and multiple stage hot presses. The cyclic cooling in single or multiple stage presses entailed a few disadvantages. There were problems in the material flow. Cyclic cooling presses must be designed so that their capacity is sufficient to cool the entire production of the hot press. The handling of the boards is complicated. Also, considerable problems are involved in the cyclic feeding of the press. The board surfaces cool at different rates, so that warping occurs.
Also, German Patent 197 50 847 has disclosed a method for cooling hot-pressed boards in which the board surface is cooled intensively in a first step within a first period t1 to about 100xc2x0 C., and in a second step the board surface is cooled with air within a second period t2 to below 60xc2x0 C. A disadvantage of this process, it is to be noted, is that moistening of the board surfaces causes the fibers and chips in the surface layers to be moistened and swell up, resulting in rough areas on the board surfaces. An additional repair operation is necessary.
German Patent 24 14 762, from which the invention sets out, discloses an apparatus which includes a continuously operating press, a sliding cooling plate system using a low specific pressure application and a gauging apparatus. The established purpose of this patent, however, is the gauging of the board strand after it leaves the continuously operating press. A sufficient rapid cooling of the board strand immediately after it leaves the continuously operating hot press thus is not possible, which means that the above-stated disadvantages in leaving the continuously operating hot press are likewise unavoidable.
The present invention provides a method for the production of boards of ligneous material comprising: forming a press material mat treated with a binding agent on a continuously moving spreading band, introducing the mat between the steel bands of a continuously operating hot press, applying pressure and heat to the mat in the continuously operating hot press, curing the mat into a board strand in the continuously operating hot press, moistening both surfaces of the hot board strand, shock cooling the hot board strand with sprayed water in a first cooling section with an evaporation zone variable in its length and adaptable to the thickness of the board strand wherein the board strand is held between endless blocking bands under multiple calendar rolls and carrying the board bands to a directly adjoining second cooling section under a predetermined light pressure through a double-band hold-down apparatus wherein the backs of the hold-down bands are sprayed with a coolant.
The present invention also provides apparatus for the production of ligneous material boards comprising: a continuously operating hot press including two flexible endless steel bands, a press table, a press ram, a first water spray cooling apparatus of variously adjustable length with water spray nozzles adapted to directly spray water onto the surfaces of the boards and calendar rolls to put the board under light pressure and a cooling apparatus, wherein the second cooling apparatus is a second double-band hold-down apparatus which can travel in a cooling range of the first water spray cooling apparatus and having a plurality of rows of coolant spray nozzles disposed on the backs of the band surfaces of the double-band hold-down apparatus which provide contact on the boards.